Hi Dave,
I was just wondering what your steering problem turned out to be, I have the same symptom combined with a knocking/grinding noise. There is side to side play in the wheel and I'm sure I can feel movement at the rack end of the track rod bush , beyond the inner rod joint. I know this seems unlikely but I'm thinking new rack?
Also, I've checked the bolts that hold the rack onto the subframe too, they seem plenty tight enough.